Diethylene glycol bis(allyl carbonate) monomer compositions are polymerized using free radical initiators to produce hard polymers. Many of these polymers are substantially transparent to visible light, are substantially colorless, have refractive indices of from about 1.48 to about 1.52, and possess a Barcol hardness above about zero. For these reasons, such monomer compositions find utility as precursors for transparent coatings, optical lenses, optical lens blanks, and other optical elements, and flat or curved transparent sheets. Light transmission characteristics may be altered by incorporating dyes, light absorbing compounds, pigments, and the like, in the monomer composition before polymerization, or by dying the polymer.
Diethylene glycol bis(allyl carbonate) monomer compositions have heretofore been produced by reacting diethylene glycol with phosgene at temperatures between 0.degree. C. and 20.degree. C. to form the corresponding dichloroformate which is then reacted with allyl alcohol in the presence of a suitable acid acceptor, as for example, pyridine, a tertiary amine, or an alkaline or alkaline earth metal hydroxide. See, for example, U.S. Pat. No. 2,403,113. Alternatively, the allyl alcohol can be reacted with phosgene and the resulting chloroformate reacted with diethylene glycol in the presence of an alkaline reagent; see, for example, U.S. Pat. No. 2,370,567. These prior diethylene glycol bis(allyl carbonate) monomer compositions are usually constituted not solely by diethylene glycol bis(allyl carbonate), per se, but also by various related coproduced compounds. The prior diethylene glycol bis(allyl carbonate) monomer compositions therefore usually comprise a mixture of diethylene glycol (allyl carbonate) compounds each represented by Formula (I): ##STR2## wherein n is a positive integer, m is a positive integer, and wherein the (allyl carbonate) compound for which the value of n is 1 constitutes from 82 to 95 area percent of the mixture of diethylene glycol (allyl carbonate) compounds. As used herein and in the claims, the relative amounts, expressed as area percent, of the diethylene glycol (allyl carbonate) compounds having differing values of n are determined by high performance liquid chromatography using an instrument equipped with a DuPont Zorbax Sil column operated at a temperature of approximately 15.degree. C., a refractive index detector, and employing a mobile phase composition of 40% methylene chloride/25% ethyl ether/25% ethyl ether which was saturated with water/10% n-hexane, where the percentages are expressed as percent by volume.
One problem associated with the polymerization of the prior diethylene glycol bis(allyl carbonate) monomer compositions is the relatively high shrinkage of the material which occurs during the course of polymerization to the final thermoset polymer. For example, there is a shrinkage of approximately 13 percent during the polymerization of diethylene glycol bis(allyl carbonate) monomer. Such high shrinkages are particularly detrimental in casting operations such as those used to produce ophthalmic lenses and ophthalmic lens blanks where the liquid monomer composition is introduced to a mold and thereafter polymerized to the final thermoset polymer.
It is known that shrinkage in the mold may be reduced by introducing a liquid prepolymer to the mold and thereafter polymerizing the prepolymer to the final thermoset polymer. The prepolymer is usually produced by partially polymerizing the diethylene glycol bis(allyl carbonate) monomer composition to consume a portion of the polymerizable ethylenic double bonds. The partial polymerization is stopped, however, before more than a trivial amount of gellation occurs so that the prepolymer may be introduced to the mold as a liquid. Examples of United States patents which disclose prepolymerization of diethylene glycol bis(allyl carbonate) monomer compositions or other polyol bis(allyl carbonate) monomer compositions include U.S. Pat. No. 4,590,248, U.S. Pat. No. 4,623,708, U.S. Pat. No. 4,686,266, U.S. Pat. No. 4,742,133, U.S. Pat. No. 4,959,429, U.S. Pat. No. 4,959,433, and U.S. Pat. No. 5,017,666.
As used herein, shrinkage is determined in accordance with the following equation: EQU S=100(D.sub.p -D.sub.m)/D.sub.p
where:
S is the shrinkage expressed as percent, PA1 D.sub.p is the density of the hard polymer (i.e., the final polymerizate), and PA1 D.sub.m is the density of the polymerizable composition which is introduced to the mold (i.e., the monomer composition or the partially polymerized monomer composition, as the case may be). PA1 I is the iodine value expressed as grams of I.sub.2 per 100 grams of sample; PA1 T.sub.b is the titer of the blank expressed as milliliters of the sodium thiosulfate solution; PA1 T.sub.s is the titer of the sample expressed as milliliters of the sodium thiosulfate solution; PA1 N is the normality of the sodium thiosulfate solution; and PA1 W is the mass of the sample expressed in grams. The ethylenic double bond utilization is calculated as follows: EQU U=100(I.sub.i -I.sub.p)/I.sub.i PA1 U is the ethylenic double bond utilization expressed as percent; PA1 I.sub.i is the iodine value of the initial monomer which has not been partially polymerized expressed as grams of I.sub.2 per 100 grams of sample; and PA1 I.sub.p is the iodine value of the partially polymerized monomer expressed as grams of I.sub.2 per 100 grams of sample. PA1 P is the peroxide assay expressed as ppm H.sub.2 O.sub.2, PA1 T.sub.s is the titer of the sample, expressed as milliliters of the sodium thiosulfate solution, PA1 T.sub.b is the titer of the blank, expressed as milliliters of the sodium thiosulfate solution, PA1 N is the normality of the sodium thiosulfate solution, and PA1 W is the mass of the sample expressed in grams. Although the peroxide content is calculated as H.sub.2 O.sub.2, for purposes of the present specification and claims, this value is taken as the hydroperoxide content. PA1 n.sub.D is the refractive index using a wavelength of 589.3 nanometers (viz., the average of the yellow doublet of sodium), PA1 n.sub.F is the refractive index using a wavelength of 486.1 nanometers (viz., the blue line of hydrogen), and PA1 n.sub.C is the refractive index using a wavelength of 656.3 nanometers (viz., the red line of hydrogen).
Low shrinkage in the mold is favored by a large utilization of polymerizable ethylenic double bonds during partial polymerization to produce the prepolymer which is introduced into the mold. However, during the partial polymerization of the diethylene glycol bis(allyl carbonate) monomer compositions, the viscosity increases as polymerizable ethylenic double bonds are consumed. For practical purposes the viscosity cannot become so high that the resulting prepolymer does not flow reasonably readily into the mold. Viscosity of the prepolymer therefore provides a practical lower limit to shrinkage in the mold.